This invention relates to a fluidized bed steam generating system and, more particularly, to such a system in which a cyclone separator is provided and is cooled by steam generated in the system.
Fluidized bed combustion systems are well known. In these arrangements, air is passed through a bed of particulate material, including a fossil fuel such as coal and an adsorbent for the sulfur released as a result of combustion of the coal, to fluidize the bed and to promote the combustion of the fuel at a relatively low temperature. Water is passed in a heat exchange relationship to the fluidized bed to generate steam. The combustion system includes a separator which separates the entrained particulate solids from the gases from the fluidized bed in the furnace section and recycles them back into the bed. This results in an attractive combination of high combustion efficiency, high sulfur adsorption, low nitrogen oxides emissions and fuel flexibility.
The most typical fluidized bed utilized in the furnace section of these type systems is commonly referred to as a "bubbling" fluidized bed in which the bed of particulate material has a relatively high density and a well-defined, or discrete, upper surface. Other types of fluidized beds utilize a "circulating" fluidized bed. According to this technique, the fluidized bed density may be below that of a typical bubbling fluidized bed, the air velocity is equal to or greater than that of a bubbling bed, and the flue gases passing through the bed entrain a substantial amount of the fine particulate solids to the extent that they are substantially saturated therewith.
Also, circulating fluidized beds are characterized by relatively high solids recycling which makes it insensitive to fuel heat release patterns, thus minimizing temperature variations, and therefore, stabilizing the emissions at a low level. The high solids recycling improves the efficiency of the mechanical device used to separate the gas from the solids for solids recycle, and the resulting increase in sulfur adsorbent and fuel residence times reduces the adsorbent and fuel consumption.
However, several problems exist in connection with these type of fluidized systems. For example, it is often necessary to add expensive cooling surfaces for superheating the steam generated in the boiler. Also, difficulties arise in controlling the temperature range of the steam generated in the system. Further, these types of beds are used in systems, such as steam generators, which include one or more cyclone separators normally provided with a hopper connected to their lower end to collect the solid particles from the separator. The separator and the hopper are usually provided with a monolithic external refractory wall which is abrasion resistant and insulative so that the outer casing runs relatively cool. However, in order to achieve proper insulation, these walls must be relatively thick which adds to the bulk, weight, and cost of the separator and hopper and require controlled, relatively long, start-up and shut down times to prevent cracking of the refractory. Also, the outside metal casing of these designs cannot be further insulated from the outside since to do so could raise its temperature as high as 1500.degree. F. which is far in excess of the maximum temperature it can tolerate. Still further, conventional separators installed in the above manner require a relatively long time to heat up before going online to eliminate premature cracking of the refractory walls, which is inconvenient and adds to the cost of the process.
Still further, systems utilizing a fluidized bed and a cyclone separator require relatively expensive, high temperature, refractory-lined ductwork and expansion joints between the fluidized bed furnace and the separator, and between the cyclone and a heat recovery section, which are fairly sophisticated and expensive.